Rapid attenuation control method and device for overhead line fault current of flexible DC power transmission system

Abstract

The invention belongs to the field of power system protection and control, and discloses an overhead line fault current rapid attenuation control method and device for a flexible direct current transmission system, and the method comprises the steps: controlling a transmitting end converter sub-module to boost when an overhead line fault signal and a transmitting end converter bridge arm output amplitude limiting signal are received, coordinating the power reference values of the fault electrode and the non-fault electrode to obtain the AC voltage reference value of the transmitting-end converter; obtaining a three-phase AC reference voltage of the target bridge arm after third harmonic injection according to the AC voltage reference value of the transmitting-end converter; according to the three-phase alternating-current reference voltage and the direct-current active damping output voltage of the transmitting-end converter, amplitude limiting is carried out on the output voltage of an active damping control link; and continuously executing the steps until the direct-current fault current of the overhead line is attenuated to zero. The method can reduce the restriction of the bridge arm output voltage range on the attenuation speed of the fault current, and accelerates the attenuation speed of the fault current of the overhead line.

Description

Technical Field

[0001] The present application relates to the technical field of power system protection and control, and in particular to a method and device for fast attenuation control of fault current of an overhead line of a flexible direct current transmission system. Background Art

[0002] As a new generation of high-voltage direct current transmission, flexible direct current transmission technology has become the core technology for realizing ultra-long-distance and ultra-large-capacity power transmission with its advantages such as independent control of active / reactive power and no risk of phase-change failure. It has shown great application potential in the fields of deep-sea wind power transmission and cross-regional power grid interconnection.

[0003] Ultra-long-distance DC transmission usually uses overhead lines for transmission, and the probability of DC faults is high. When a DC short-circuit fault occurs in the overhead line of the ultra-long-distance flexible DC transmission system, a large amount of energy is contained in the long-distance line, resulting in a long time for the DC fault current to be cleared. Adding DC active damping control can speed up the decay of the fault current. However, due to the physical constraints of the negative level output range of the modular multilevel converter (MMC) bridge arm, the converter output is limited, resulting in a slow decay of the DC fault current and easily causing serious overvoltage of the submodule capacitor.

[0004] Therefore, it is necessary to develop a fault current rapid attenuation control method to improve the fault current attenuation speed while alleviating the sub-module voltage stress. Summary of the invention

[0005] In order to solve the above problems, the present invention provides a method and device for controlling the rapid attenuation of fault current of an overhead line of a flexible DC transmission system, which is used to reduce the limitation of the physical constraint of the negative level output range of the converter bridge arm on the rapid attenuation of the fault current during the overhead line fault crossing process of the ultra-long distance flexible DC transmission system, maximize the utilization of the bridge arm output voltage space, accelerate the attenuation speed of the overhead line fault current, and alleviate the voltage stress of the sub-module.

[0006] According to a first aspect of an embodiment of the present application, a method for controlling rapid attenuation of an overhead line fault current in a flexible direct current transmission system is provided, comprising: S1: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The reference value of the shaft AC voltage increases the voltage reference value of the modulation link; S2: When receiving the overhead line fault signal and the output limit signal of the bridge arm of the sending-end converter, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference value; S3: According to the sending end converterd Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected; S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; S5: Continue to execute S1-S4 until the DC fault current of the overhead line decays to zero.

[0007] According to a second aspect of an embodiment of the present application, there is provided an overhead line fault current rapid attenuation control device for a flexible direct current transmission system, comprising: The submodule boost control module is used to control the boost of the sending-end converter submodule when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, and obtain the sending-end converter after the voltage and current double-loop PI control. d The reference value of the shaft AC voltage increases the voltage reference value of the modulation link; The power coordination control module of the fault pole and the non-fault pole is used to coordinate the power reference values ​​of the fault pole and the non-fault pole according to the power reference values ​​of the fault pole and the non-fault pole when receiving the overhead line fault signal and the output limiting signal of the bridge arm of the sending-end converter, and obtain the sending-end converter after the power current double-loop PI control. q Shaft AC voltage reference value; The third harmonic injection control module is used to control the sending end converter according to the d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected; A DC active damping limiting control module is used to limit the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; The fault current attenuation control module is used to continuously execute the submodule boost control module-DC active damping limit control module until the overhead line DC fault current decays to zero.

[0008] According to a third aspect of an embodiment of the present application, there is provided an electronic device, including: one or more processors; A memory for storing one or more programs; When the one or more programs are executed by the one or more processors, the one or more processors implement the method as described in the first aspect.

[0009] According to a fourth aspect of an embodiment of the present application, a computer-readable storage medium is provided, on which computer instructions are stored, and when the instructions are executed by a processor, the steps of the method described in the first aspect are implemented.

[0010] The technical solution provided by the embodiments of the present application may have the following beneficial effects: It can be seen from the above embodiments that the present invention reduces the amplitude of the three-phase AC reference voltage by injecting the third harmonic when calculating the three-phase AC reference voltage during the overhead line DC fault ride-through, reduces the restriction of the bridge arm output voltage range on the bridge arm output negative voltage level, and thus accelerates the fault current decay speed; the present invention reduces the amplitude of the three-phase AC reference voltage by coordinating the power reference values ​​of the fault pole and the non-fault pole, and at the same time reduces the power transmission shortage of the sending-end converter during the DC fault, thereby avoiding the instability of the flexible DC transmission system; the present invention widens the output voltage range of the bridge arm of the sending-end converter by controlling the boost of the sending-end converter submodule and increasing the voltage reference value of the modulation link, so that the bridge arm outputs more negative levels, thereby accelerating the fault current decay speed; the present invention reduces the restriction of the bridge arm output voltage range by calculating and applying the optimal active damping control limit value, thereby accelerating the fault current decay speed and avoiding submodule capacitor overvoltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a flow chart of a method for fast attenuation control of overhead line fault current in a flexible direct current transmission system provided by an embodiment of the present invention.

[0012] Figure 2 It is a block diagram of a method for fast attenuation control of overhead line fault current in a flexible direct current transmission system provided by an embodiment of the present invention.

[0013] Figure 3 It is a flow chart of the submodule boost control strategy provided by an embodiment of the present invention.

[0014] Figure 4 It is a flow chart of the power coordination control strategy of the fault pole and the non-fault pole provided by the embodiment of the present invention.

[0015] Figure 5 It is a flow chart of the third harmonic injection strategy provided by an embodiment of the present invention.

[0016] Figure 6 This is a waveform diagram of a DC fault current for fast attenuation control of a fault current provided by an embodiment of the present invention.

[0017] Figure 7 The invention provides a fault current fast decay control sending-end converter. a Phase AC voltage reference value waveform.

[0018] Figure 8The invention provides a fault current fast decay control sending-end converter. a Average switching function waveform of the phase bridge arm.

[0019] Fig. 9 It is a block diagram of a device for rapidly attenuating fault current of an overhead line of a flexible direct current transmission system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0020] Here, exemplary embodiments will be described in detail.

[0021] Figure 1 FIG. 1 is a flow chart showing a method for controlling the rapid attenuation of fault current in an overhead line of a flexible direct current transmission system according to an exemplary embodiment. Figure 1 As shown, the method may include the following steps: S1: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The reference value of the shaft AC voltage increases the voltage reference value of the modulation link; S2: When receiving the overhead line fault signal and the output limit signal of the bridge arm of the sending-end converter, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference value; S3: According to the sending end converter d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected; S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; S5: Continue to execute S1-S4 until the DC fault current of the overhead line decays to zero.

[0022] It can be seen from the above embodiments that the present invention reduces the amplitude of the three-phase AC reference voltage by injecting the third harmonic when calculating the three-phase AC reference voltage during the overhead line DC fault ride-through, reduces the restriction of the bridge arm output voltage range on the bridge arm output negative voltage level, and thus accelerates the fault current decay speed; the present invention reduces the amplitude of the three-phase AC reference voltage by coordinating the power reference values ​​of the fault pole and the non-fault pole, and at the same time reduces the power transmission shortage of the sending-end converter during the DC fault, thereby avoiding the instability of the flexible DC transmission system; the present invention widens the output voltage range of the bridge arm of the sending-end converter by controlling the boost of the sending-end converter submodule and increasing the voltage reference value of the modulation link, so that the bridge arm outputs more negative levels, thereby accelerating the fault current decay speed; the present invention reduces the restriction of the bridge arm output voltage range by calculating and applying the optimal active damping control limit value, thereby accelerating the fault current decay speed and avoiding submodule capacitor overvoltage.

[0023] It should be noted that in the specific implementation, the overhead line DC short-circuit fault signal is given by the fault detection link, and the output limiting signal of the bridge arm of the sending-end converter is obtained by real-time detection of the bridge arm reference voltage. V dcn / 2 or less than - V dcn / 2 is considered that the output of the bridge arm of the sending-end converter is limited, where V dcn is the rated DC voltage of the sending-end converter. At this time, the output limiting signal of the bridge arm of the sending-end converter is given, and the rapid attenuation control of the overhead line fault current of the flexible DC transmission system needs to be started.

[0024] Figure 2 This is a block diagram of a method for rapidly attenuating fault current in an overhead line of a flexible direct current transmission system provided by an embodiment of the present invention. Each step is described in detail below.

[0025] In the specific implementation of S1: when receiving the overhead line fault signal and the output limit signal of the bridge arm of the sending-end converter, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the modulation link voltage reference value; this step includes the following sub-steps: S11: Control the voltage boost of the sending-end converter submodule to increase the reference value of the average voltage of the sending-end converter submodule capacitor, and obtain the sending-end converter after voltage and current dual-loop PI control. d Shaft AC voltage reference value; Specifically, Figure 3 FIG. 1 is a flow chart showing a submodule boost control in an overhead line fault current rapid decay control strategy of a flexible direct current transmission system according to an exemplary embodiment. Figure 3As shown, when receiving the overhead line fault signal and the output limit signal of the bridge arm of the sending-end converter, the reference value of the average capacitor voltage of the sending-end converter submodule is switched to V cref1 , and improves the reference value of the average capacitor voltage of the submodule. The error between the reference value of the average capacitor voltage of the sending-end converter submodule and the actual value of the average capacitor voltage of the sending-end converter submodule is used as the input value of the voltage loop PI controller. According to the output of the voltage loop PI controller, the sending-end converter AC d The shaft voltage and decoupling compensation are used to obtain the AC current of the sending-end converter. d The shaft voltage reference value. The above process can be expressed by the following formula: ; ; ; In the formula, V cref1 is the reference value of the capacitor voltage of the sending-end converter submodule during fault ride-through; V cref0 is the initial reference value of the capacitor voltage of the sending-end converter submodule; k c is the submodule capacitor boost coefficient, k c >1; I dref For the sending end converter d Shaft AC current reference value; K p1 , K i1 The proportional coefficient and integral coefficient of the submodule voltage loop PI controller; p is a differential operator; V c1 is the average voltage of the capacitor of the sending-end converter submodule; V dref1 For the sending end converter d Shaft AC voltage reference value; K p3 , K i3 for d Proportional coefficient and integral coefficient of the axis current loop PI controller; I d For the sending end converter d Shaft AC current; ω is the angular frequency of the AC side of the sending-end converter; L is the equivalent inductance of the AC circuit of the sending-end converter; I q For the sending end converter q Shaft AC current; Vsd For the AC side of the sending - end converter d axis voltage.

[0026] In the boosting control of the sub - modules of the sending - end converter, the boosting coefficient of the sub - modules k c > 1, and its upper limit is determined by the multiple of the short - time over - voltage operation that the sub - modules can withstand. For example, it can be taken as k c = 1.3.

[0027] S12: Calculate the maximum value of the bridge - arm output voltage according to the reference value of the average voltage of the capacitors of the sub - modules of the sending - end converter, and perform capacitor modulation with the maximum value of the bridge - arm output voltage as the voltage reference; Specifically, the sending - end converter of the ultra - long - distance flexible DC transmission system is a modular multilevel converter, and the modulation process generally adopts direct modulation. When the boosting control of the sub - modules of the sending - end converter starts, calculate the maximum value of the bridge - arm output voltage of the sending - end converter according to the reference value of the average voltage of the capacitors of the sub - modules of the sending - end converter. In the generation link of the sub - module modulation signal, use the maximum value of the bridge - arm output voltage as the voltage reference to generate the average switching function of the bridge - arm of the sending - end converter. The above process can be expressed by the following formula: In the formula, V N is the calculated maximum value of the bridge - arm output voltage of the sending - end converter; N is the number of sub - modules in the bridge - arm of the sending - end converter; V cref1 is the reference value of the capacitor voltage of the sub - modules of the sending - end converter during fault ride - through; S jp1 , S jn1 are the average switching functions of the upper and lower bridge - arms of the sending - end converter during fault ride - through; V jref1 is the reference value of the three - phase AC voltage of the sending - end converter, where abc represents j three - phase; abc V cirj is the output voltage of the circulating - current suppression link of the sending - end converter; V vir is the output voltage of the DC active damping control link of the sending - end converter.

[0028] ​It should be noted that here, by controlling the boost of the sub-modules of the sending-end converter and adopting an increased maximum value of the arm output voltage in the sub-module modulation signal generation link, the amplitude of the average switching function of the arm is reduced, and the limitation of the arm output voltage range of the sending-end converter on the decay rate of the fault current is reduced. The comparison of the average switching function of the sending-end converter arm between the traditional fault ride-through control and the fault ride-through control of this embodiment can be expressed by the following formula: In the formula, S jk0 is the average switching function of the sending-end converter arm during traditional fault ride-through; S jk1 is the average switching function of the sending-end converter arm during the fault ride-through of this embodiment; V jref1 is the reference value of the three-phase AC voltage of the sending-end converter abc where j represents abc three phases; V cirj is the output voltage of the circulating current suppression link of the sending-end converter; V vir is the output voltage of the DC active damping control link of the sending-end converter; V cref0 is the reference value of the capacitor voltage of the sub-module of the sending-end converter during traditional fault ride-through; V cref1 is the reference value of the capacitor voltage of the sub-module of the sending-end converter during the fault ride-through of this embodiment.

[0029] In this embodiment, by increasing the average capacitor voltage of the sub-modules of the sending-end converter during fault ride-through, the voltage range of the arm output is expanded, and the sending-end converter arm can output more negative levels. Furthermore, the physical constraint of the arm output voltage on the arm output voltage range is reduced, achieving the goal of accelerating the decay rate of the DC fault current on the overhead line.

[0030] In the specific implementation of S2: when receiving the overhead line fault signal and the arm output limiting signal of the sending-end converter, according to the power reference values of the faulty pole and the non-faulty pole, coordinate the power reference values of the faulty pole and the non-faulty pole, and obtain the reference value of the α-axis AC voltage of the sending-end converter after power-current double-loop PI control; referring to q this step includes the following sub-steps: Figure 4 S21: According to the active power reference values and reactive power reference values of the faulty pole and the non-faulty pole, within the power limit range of the converter, reduce the reactive power reference value of the faulty pole, reduce the reactive power reference value of the non-faulty pole, and increase the active power reference value of the non-faulty pole; ​Specifically, when receiving the overhead line fault signal and the sending - end converter bridge arm output limit signal, according to the power reference values of the faulty pole and the non - faulty pole, coordinate the power reference values of the faulty pole and the non - faulty pole, and actively regulate the active and reactive power transmitted by the faulty pole and the non - faulty pole. Reduce the reactive power reference value of the non - faulty pole of the converter, increase the active power reference value of the non - faulty pole of the converter, and increase the reactive power reference value of the faulty pole of the converter. The comparison of the reactive power of the non - faulty pole, the active power of the non - faulty pole, and the reactive power of the faulty pole before fault crossing and after the start of the fault current rapid decay control strategy can be expressed by the following formula: In the formula, Q acpref0 、 Q acpref1 are the reactive power reference values of the non - faulty pole before fault crossing and after the start of the fault current rapid decay control strategy respectively; P acpref0 、 P acpref1 are the active power reference values of the non - faulty pole before fault crossing and after the start of the fault current rapid decay control strategy respectively; Q acnref0 、 Q acnref1 are the reactive power reference values of the faulty pole before fault crossing and after the start of the fault current rapid decay control strategy respectively.

[0031] It should be noted that the reactive power reference value of the positive pole of the sending - end converter is generally positive, the reactive power of the negative pole is generally negative, and the active power of the positive and negative poles is positive. The Q acpref0 、 Q acpref1 、 P acpref0 、 P acpref1 、 Q acnref0 、 Q acnref1 shown here are all numerical values without positive or negative signs.

[0032] S22: Obtain the reference value of the q axis AC voltage of the sending - end converter after power - current double - loop PI control; Specifically, obtain the reference value of the q axis AC voltage of the sending - end converter after power - current double - loop PI control; At this time, the AC q axis current model of the faulty pole of the sending - end converter and the comparison of the relationship before and after the fault are expressed by the formula as follows: In the formula, I qn0 、I qn1 is the AC q axis current of the sending - end converter before and after the start of the power coordination control strategy between the faulty pole and the non - faulty pole in this embodiment; Q acn1 is the reactive power of the faulty pole of the sending - end converter after the start of the fast - decaying fault - current control strategy; V sd is the AC d axis voltage of the sending - end converter.

[0033] It should be noted that the reactive power of the negative pole of the sending - end converter is generally negative, the AC q - axis current of the sending - end converter is negative, and the AC q axis current of the sending - end converter decreases after the start of the fast - decaying fault - current control strategy.

[0034] The model of the axis - current loop equation of the faulty pole of the sending - end converter and the comparison of the relationship before and after the fault are expressed by the following formula: d In the formula, In the formula, V dref0 , V dref1 are the reference values of the AC d axis voltage of the sending - end converter before and after the start of the fast - decaying fault - current control strategy in this embodiment; K p3 , K i3 are the proportional coefficient and integral coefficient of the PI controller of the d axis - current loop; p is the differential operator; I d is the AC d axis current of the sending - end converter; I dref0 , I dref1 are the reference values of the AC d axis current of the sending - end converter before and after the start of the fast - decaying fault - current control strategy in this embodiment; ω is the angular frequency of the AC side of the sending - end converter; L is the equivalent inductance of the AC circuit of the sending - end converter; I qn0 , I qn1 are the AC q axis currents of the sending - end converter before and after the start of the fast - decaying fault - current control strategy in this embodiment; Vsd For the AC of the sending - end converter d axis voltage.

[0035] It should be noted that in this embodiment, the coordination of the active power and reactive power of the faulty pole and non - faulty pole of the sending - end converter follows the following requirements: the reference value of the reactive power of the faulty pole increases, but is less than the reactive - power transmission limit of the faulty pole; the reference value of the reactive power of the non - faulty pole decreases, and in the most extreme case, the reference value of the reactive power of the non - faulty pole is zero; the reference value of the active power of the non - faulty pole increases, and it is required that the total transmission power of the non - faulty pole is less than the transmission capacity.

[0036] In this embodiment, by coordinating the power reference values of the faulty pole and non - faulty pole of the sending - end converter during the fault - ride - through period, the AC q axis current of the faulty pole of the sending - end converter is reduced, the overall amplitude of the three - phase AC voltage reference value is decreased, so that the sending - end converter can increase the output magnitude of the DC negative level, enabling the converter bridge arm to output more negative levels, and further achieving the goal of accelerating the decay rate of the DC fault current in the overhead line. At the same time, the active power transmitted by the non - faulty pole of the sending - end converter increases, which helps to stabilize the flexible DC transmission system and greatly reduces the risk of system instability.

[0037] In the specific implementation of S3: According to the sending - end converter d axis AC voltage reference value and q axis AC voltage reference value, obtain the amplitude and phase of the AC voltage reference value of the sending - end converter, and determine the target three - phase AC reference voltage of the sending - end converter after the injection of the third - harmonic. Refer to Figure 5 , this step includes the following sub - steps: S31: According to the sending - end converter d axis AC voltage reference value and q axis AC voltage reference value, obtain the amplitude and phase of the three - phase AC voltage through trigonometric function relationships; Specifically, according to the AC dq axis reference voltage of the sending - end converter obtained in the previous step, through the Park inverse transformation, obtain the abc AC reference voltage in the three - phase stationary coordinate system. Obtain the amplitude and phase of the three - phase AC reference voltage through trigonometric function relationships. The formula expressions of the above - mentioned process are as follows: In the formula, V m1 is the amplitude of the AC reference voltage of the sending - end converter; θ j1 is the phase of the AC reference voltage; V aref1 、 V bref1、V cref1 For abc the three-phase AC reference voltage; f indicating the calculation process of the trigonometric function relationship.

[0038] f The formula indicating the calculation process of the trigonometric function relationship is as follows: In the formula, V m1 is the amplitude of the AC reference voltage of the sending-end converter; V iref1 is abc the three-phase AC reference voltage; θ j1 is the phase of the AC reference voltage; V jref1 is abc the three-phase AC reference voltage.

[0039] S32: Calculate the optimal amplitude and phase of the target third harmonic according to the amplitude and phase of the three-phase AC voltage; Specifically, according to the calculated amplitude and phase of the three-phase AC reference voltage, calculate the optimal amplitude and phase of the target third harmonic to be injected. The formula is as follows: In the formula, V m3 is the amplitude of the target third harmonic voltage to be injected; V m1 is the amplitude of the AC reference voltage of the sending-end converter; θ j3 is the phase of the target third harmonic voltage to be injected; θ j1 is the phase of the AC reference voltage of the sending-end converter; V jref3 is the target third harmonic voltage to be injected.

[0040] It should be noted that the amplitude of the target third harmonic voltage to be injected V m3 is obtained through mathematical calculation, and the requirement it meets is that after the third harmonic voltage is superimposed on the original AC reference voltage, the minimum value of the overall waveform reaches the maximum, and the amplitude V m3 .

[0041] S33: Calculate the target AC reference voltage of the sending-end converter based on the optimal amplitude and phase of the target third harmonic and the superposition of the AC voltage reference value and the third harmonic.

[0042] Specifically, determine the amplitude and phase of the target three-phase AC reference voltage of the sending-end converter. Superimpose the target third-harmonic voltage to be injected on the AC reference voltage in the three-phase stationary coordinate system to obtain and input the target three-phase AC reference voltage of the sending-end converter after the third-harmonic injection. The formula is as follows: abc In the formula, where, V jref is the target three-phase AC reference voltage after the third-harmonic injection; V m3 is the amplitude of the target third-harmonic voltage to be injected; V m1 is the amplitude of the AC reference voltage of the sending-end converter; θ j3 is the phase of the target third-harmonic voltage to be injected; θ j1 is the phase of the AC reference voltage of the sending-end converter.

[0043] In this embodiment, during the fault ride-through period, the target three-phase AC reference voltage of the sending-end converter is generated through third-harmonic injection, so that the overall amplitude of the three-phase AC reference voltage of the sending-end converter is reduced, thereby enabling the sending-end converter to have more level spaces and being able to be used for negative-level output. The sending-end converter increases the negative-level output, and further achieves the goal of accelerating the decay rate of the DC fault current on the overhead line.

[0044] In the specific implementation of S4: Limit the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; this step includes the following sub-steps: S41: Calculate the optimal active damping control limit value according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; Calculate the optimal limit value of the DC active damping control link according to the output voltage of the DC active damping control link and the target three-phase AC reference voltage of the sending-end converter obtained in the foregoing links.

[0045] During the fault ride-through period, the output voltage of the bridge arm is restricted by the physical constraint of the bridge arm output range. In order to satisfy the simultaneous control of the AC / DC loop of the sending-end converter, the bridge arm output voltage range needs to satisfy the output requirements of the target AC reference voltage and the DC active damping output voltage at the same time. By the method of coordinating the target AC reference voltage and the DC active damping control, the bridge arm output voltage space can be maximally utilized, and the influence of the bridge arm output limit on the control performance of the sending-end converter can be reduced. The formula for the output limit design of the active damping control link is as follows: Wherein, V jkref ’ is the output reference voltage of the target arm after adding DC active damping control; V jkref is the target three-phase AC reference voltage of the sending-end converter; V cirj is the voltage output by the circulating current suppression link; V vir is the output voltage of the DC active damping control; V virlim is the output limit value of the DC active damping control; V virmin is the minimum value of the output voltage of the DC active damping control, which can be designed to be -1.1 V m1 , V m1 is the amplitude of the AC reference voltage of the sending-end converter; V dcn is the rated DC voltage at the DC end of the sending-end converter.

[0046] S42: Limit the output of the active damping control link according to the optimal active damping control limit value.

[0047] In this embodiment, the target AC reference voltage and the DC active damping control are coordinated during the fault ride-through period, so that the output reference voltage of the sending-end converter arm is reduced within the arm output range, maximizing the utilization of the arm output voltage range, improving the control effect of the AC-DC loop of the sending-end converter, accelerating the decay speed of the overhead line DC fault current, and not causing serious overvoltage of the sub-module capacitor.

[0048] The parameters of the ultra-long-distance flexible DC transmission system are shown in Table 1.

[0049] Table 1: The effectiveness of the overhead line fault current rapid decay control method using the flexible DC transmission system is verified in the following aspects: reducing the amplitude of the AC reference voltage, improving the utilization degree of the arm output voltage, and accelerating the decay speed of the overhead line fault current.

[0050] When using the overhead line fault current rapid decay control of the flexible DC transmission system, the fault current, the a phase AC voltage reference value of the sending-end converter, the a phase arm average switching function waveform of the sending-end converter is as Figure 6 , Figure 7 , Figure 8 As shown. The fault current decay time of the traditional fault ride-through control is 4.88 s, and the fault current decay time of the overhead line fault current rapid decay control of the VSC-HVDC system is 132 ms, and the fault current decay speed is increased by 37 times; the negative amplitude of the reference voltage of the sending converter bridge arm of the traditional fault ride-through control is 329 kV, and the negative amplitude of the reference voltage of the sending converter bridge arm of the overhead line fault current rapid decay control of the VSC-HVDC system is 292 kV, and the amplitude of the reference voltage of the sending converter bridge arm is reduced by 11.3%; the modulation degree of the sending converter bridge arm voltage of the traditional fault ride-through control is 0.823, and the modulation degree of the sending converter bridge arm voltage of the overhead line fault current rapid decay control of the VSC-HVDC system is 0.664, and the modulation degree of the sending converter bridge arm voltage is reduced by 19.3%. Under the control strategy of the present invention, the average switching function of the sending converter bridge arm has been controlled within the limit range of the output voltage of the sending converter bridge arm, the reference voltage of the sending converter bridge arm is reduced by the constraint of the output voltage limit of the bridge arm, and the sending converter as a whole can output more negative levels, thereby accelerating the decay speed of the DC fault current. The overhead line fault current rapid decay control of the VSC-HVDC system significantly improves the utilization degree of the bridge arm output voltage and significantly accelerates the decay speed of the overhead line fault current.

[0051] Corresponding to the embodiment of the overhead line fault current rapid decay control method of the VSC-HVDC system described above, the present application also provides an embodiment of an overhead line fault current rapid decay control device for a VSC-HVDC system.

[0052] Fig. 9 FIG. is a block diagram of an overhead line fault current rapid decay control device for a VSC-HVDC system shown according to an exemplary embodiment. Referring to Fig. 9 , the device includes: Sub-module boost control module 1, configured to control the boost of the sending converter sub-module when receiving an overhead line fault signal and a sending converter bridge arm output limit signal, and obtain the sending converter d axis AC voltage reference value after voltage-current double-loop PI control, and increase the voltage reference value of the modulation link; Fault pole and non-fault pole power coordination control module 2, configured to coordinate the power reference values of the fault pole and the non-fault pole according to the fault pole and non-fault pole power reference values when receiving an overhead line fault signal and a sending converter bridge arm output limit signal, and obtain the sending converter q axis AC voltage reference value after power-current double-loop PI control; Third harmonic injection control module 3, configured to obtain the amplitude and phase of the sending converter AC voltage reference value according to the sending converter d axis AC voltage reference value and q axis AC voltage reference value, and determine the target three-phase AC reference voltage of the sending converter after third harmonic injection; A DC active damping and amplitude limiting control module 4, configured to limit the amplitude of the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; A fault current decay control module 5, configured to continuously execute the sub-module boost control module - DC active damping and amplitude limiting control module until the DC fault current of the overhead line decays to zero.

[0053] Regarding the device in the above embodiments, the specific manners in which each module performs operations have been described in detail in the embodiments related to the method, and will not be elaborated herein.

[0054] For the device embodiments, since they basically correspond to the method embodiments, the relevant parts can refer to the partial descriptions of the method embodiments. The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this application. Those of ordinary skill in the art can understand and implement it without creative efforts.

[0055] Correspondingly, this application also provides an electronic device, including: one or more processors; a memory for storing one or more programs; when the one or more programs are executed by the one or more processors, the one or more processors implement the method for rapidly decaying the fault current of the overhead line in the flexible DC transmission system as described above.

[0056] Correspondingly, this application also provides a computer-readable storage medium, on which computer instructions are stored, and when the instructions are executed by a processor, the method for rapidly decaying the fault current of the overhead line in the flexible DC transmission system as described above is implemented.

[0057] After considering the specification and practicing the content disclosed herein, those skilled in the art will readily think of other implementation manners of this application. This application aims to cover any variations, uses, or adaptive changes of this application, and these variations, uses, or adaptive changes follow the general principles of this application and include the common general knowledge or conventional technical means in the technical field not disclosed in this application. The specification and embodiments are only regarded as exemplary, and the true scope and spirit of this application are pointed out by the claims.

[0058] It should be understood that this application is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of this application is only limited by the appended claims.

Claims

1. A method for fast attenuation control of overhead line fault current in a flexible direct current transmission system, characterized in that: include: S1: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The reference value of the shaft AC voltage increases the voltage reference value of the modulation link; S2: When receiving the overhead line fault signal and the output limit signal of the bridge arm of the sending-end converter, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference value; S3: According to the sending end converter d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected; S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; S5: Continue to execute S1-S4 until the DC fault current of the overhead line decays to zero.

2. The method according to claim 1, characterized in that Control the boost voltage of the sending-end converter submodule, and obtain the sending-end converter after voltage and current dual-loop PI control d The shaft AC voltage reference value increases the voltage reference value of the modulation link, including: Control the boost voltage of the sending-end converter submodule to increase the reference value of the average voltage of the sending-end converter submodule capacitor, and obtain the sending-end converter after voltage and current dual-loop PI control. d Shaft AC voltage reference value; The maximum value of the bridge arm output voltage is calculated according to the reference value of the average voltage of the capacitor of the sending-end converter submodule, and the capacitor modulation is performed with the maximum value of the bridge arm output voltage as the voltage reference.

3. The method according to claim 1, characterized in that: According to the power reference values ​​of the fault pole and the non-fault pole, the power reference values ​​of the fault pole and the non-fault pole are coordinated, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference values, including: According to the active power reference value and reactive power reference value of the fault pole and the non-fault pole, within the power limit range of the converter, the reactive power reference value of the fault pole is reduced, the reactive power reference value of the non-fault pole is reduced, and the active power reference value of the non-fault pole is increased; After the power and current double-loop PI control, the sending-end converter is obtained q Shaft AC voltage reference value.

4. The method according to claim 1, characterized in that According to the sending-end converter d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and the target three-phase AC reference voltage of the sending-end converter after the third harmonic injection is determined, including: According to the sending-end converter d Shaft AC voltage reference and q The shaft AC voltage reference value, the amplitude and phase of the three-phase AC voltage are obtained through the trigonometric function relationship; Calculate the optimal amplitude and phase of the target third harmonic according to the amplitude and phase of the three-phase AC voltage; According to the optimal amplitude and phase of the third harmonic, the target three-phase AC reference voltage of the sending-end converter based on the AC voltage reference value and the superposition of the third harmonic is calculated.

5. The method according to claim 1, characterized in that According to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage, the output voltage of the active damping control link is limited, including: Calculating an optimal active damping control limit value according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; According to the optimal active damping control limiting value, the output of the active damping control link is limited.

6. A device for controlling the rapid attenuation of fault current in overhead lines of a flexible direct current transmission system, characterized in that: include: The submodule boost control module is used to control the boost of the sending-end converter submodule when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, and obtain the sending-end converter after the voltage and current double-loop PI control. d The reference value of the shaft AC voltage increases the voltage reference value of the modulation link; The power coordination control module of the fault pole and the non-fault pole is used to coordinate the power reference values ​​of the fault pole and the non-fault pole according to the power reference values ​​of the fault pole and the non-fault pole when receiving the overhead line fault signal and the output limiting signal of the bridge arm of the sending-end converter, and obtain the sending-end converter after the power current double-loop PI control. q Shaft AC voltage reference value; The third harmonic injection control module is used to control the sending end converter according to the d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected; A DC active damping limiting control module is used to limit the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; The fault current attenuation control module is used to continuously execute the submodule boost control module-DC active damping limit control module until the DC fault current of the overhead line decays to zero.

7. An electronic device, characterized in that: include: one or more processors; A memory for storing one or more programs; When the one or more programs are executed by the one or more processors, the one or more processors implement the method according to any one of claims 1 to 5.

8. A computer-readable storage medium having computer instructions stored thereon, characterized in that: When the instruction is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

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